What is MYSQL Partitioning?

The idea behind partitioning isn't to use multiple servers but to use multiple tables instead of one table. You can divide a table into many tables so that you can have old data in one sub table and new data in another table. Then the database can optimize queries where you ask for new data knowing that they are in the second table. What's more, you define how the data is partitioned.

Simple example from the MySQL Documentation:

CREATE TABLE employees (
    id INT NOT NULL,
    fname VARCHAR(30),
    lname VARCHAR(30),
    hired DATE NOT NULL DEFAULT '1970-01-01',
    separated DATE NOT NULL DEFAULT '9999-12-31',
    job_code INT,
    store_id INT
)
PARTITION BY RANGE ( YEAR(separated) ) (
    PARTITION p0 VALUES LESS THAN (1991),
    PARTITION p1 VALUES LESS THAN (1996),
    PARTITION p2 VALUES LESS THAN (2001),
    PARTITION p3 VALUES LESS THAN MAXVALUE
);

This allows to speed up e.g.:

1. Dropping old data by simple:

   ALTER TABLE employees DROP PARTITION p0;
   

2. Database can speed up a query like this:

   SELECT COUNT(*)
   FROM employees
   WHERE separated BETWEEN '2000-01-01' AND '2000-12-31'
   GROUP BY store_id;
   

Knowing that all data is stored only on the p2 partition.

A partitioned table is a single logical table that’s composed of multiple physical subtables. The partitioning code is really just a wrapper around a set of Handler objects that represent the underlying partitions, and it forwards requests to the storage engine through the Handler objects. Partitioning is a kind of black box that hides the underlying partitions from you at the SQL layer, although you can see them quite easily by looking at the filesystem, where you’ll see the component tables with a hash-delimited naming convention.

For example, here’s a simple way to place each year’s worth of sales into a separate partition:

CREATE TABLE sales (
 order_date DATETIME NOT NULL,
 -- Other columns omitted
) ENGINE=InnoDB PARTITION BY RANGE(YEAR(order_date)) (
 PARTITION p_2010 VALUES LESS THAN (2010),
 PARTITION p_2011 VALUES LESS THAN (2011),
 PARTITION p_2012 VALUES LESS THAN (2012),
 PARTITION p_catchall VALUES LESS THAN MAXVALUE );

read more here.